G. F. Petrie and C. R, Avari 
323 
Summary of Conclusions. 
The above-mentioned considerations bearing upon the influence of 
insect prevalence and the influence of meteorological conditions on the 
prevalence of trypanosome infections in rats may be summarized as 
follows: 
(1) There are definite seasonal variations in the prevalence of try¬ 
panosome infections in M. rattus and in M. decumanus. 
(2) No definite correlation is forthcoming between the seasonal 
prevalence of T. lewisi and the seasonal prevalence of rat fleas. The 
seasonal prevalence of the transmitting insects is apparently a factor of 
subsidiary importance in the causation of the trypanosome prevalence. 
(3) The evidence adduced above indicates that the dominant factor 
determining the seasonal prevalence of trypanosome infections is the 
atmospheric temperature, the optimum temperature approximating to 
79° F. It is probable that temperature operates by influencing a 
developmental cycle of the trypanosomes in the transmitting insects. 
(4) It is possible that in addition to temperature the atmospheric 
humidity may play a part by influencing the direct or mechanical 
transference of the infection. 
It may be added in conclusion that the considerations discussed 
above seem to us to indicate strongly that transmission experiments 
with T. lewisi (and probably other trypanosomes) will give the best 
chance of success if carried out during the season of greatest pre¬ 
valence of the natural infection, and further that in experimental 
investigations of the development of trypanosomes—pathogenic or 
non-pathogenic—in insects transmitting the infection to man or animals, 
the temperature at which the experiments are carried out should be 
looked upon as of the greatest importance. 
Moreover, since it has been shown above that the seasonal prevalence 
of another parasite of the rat (a Leucocytozoon) bears a similar relation 
to temperature it is probable that the statement just made is a general¬ 
isation which will be found applicable to many protozoal infections 
transmitted by the agency of insects. 
We have pleasure in recording our indebtedness to Lt.-Col. W. B. 
Bannerman, M.D., I.M.S., Director of the Bombay Plague Research 
Laboratory at the time these observations were made, and to Dr V. L. 
Manker, Mr P. S. Ramachandrier, Mr Mallet and Mr Sabnis for assist¬ 
ance rendered to us in various ways. 
